Three Mile Island, United States (1979)

Three Mile Island, ranking #4, involved a partial meltdown of Reactor 2 due to a stuck-open relief valve and confusing control room indicators, releasing minimal radioactive gas with no direct fatalities—radiation exposure was 1 million times less than Chernobyl’s. The event halted new U.S. nuclear plant construction for over 30 years, a regulatory impact that outperforms any other incident on this list, including Kyshtym’s secrecy. Cleanup cost $1 billion, significantly cheaper than Fukushima’s $200 billion, but the psychological and policy effects transformed global nuclear safety standards, cementing its position despite the low health toll.